53 Pa. Stat. § 22674
The commission shall, from time to time and without unreasonable delay, invest all balances remaining to the credit of the sinking funds, not immediately applicable to the extinguishment of the city debt, in any proper bonds issued by such city, the United States, State of Pennsylvania, and not otherwise. Said commission shall, as and when it deems it necessary, have the right to sell or exchange at the market rate any bonds so purchased: Provided, That no bonds so purchased shall be sold at less than par value and accrued interest, unless said commission shall be authorized so to do by an ordinance of the council, and the council shall appropriate and pay over to such Sinking Fund Commission any such difference between the selling price thereof and par value of accrued interest.
53 P.S. § 22674